This invention relates to digital communication systems and, more particularly, to a processor arrangement employable in such systems for increasing the run length of digital signals.
In conventional facsimile systems, a picture image includes a plurality of lines, each line having a plurality of picture elements. Usually, within a facsimile system transmitter, a coder digitally encodes a voltage proportional in amplitude to the level of brightness of a picture element (pel). The encoded voltage, hereinafter called the pel signal, is transmitted to a receiver where it is decoded and a facsimile of the picture image assembled.
In facsimile systems using predictive coding, a predictor error signal, rather than the current pel signal, is so transmitted. One method of predictive coding is disclosed in J. S. Wholey, "The Coding of Pictorial Data," IRE Transactions on Information Theory, Vol. IT-7, No. 2 (April, 1961), pp 99-104. According to the prior art, a survey of pictures representative of the class of pictures to be coded is made. One class might include pictures of circuit diagrams; a second class might include pictures of single-spaced, typed manuscripts, etc. The purpose of the survey is to ascertain for each class the relative frequency that a pel signal will be a particular level of brightness, given the brightness level of each pel in a neighborhood thereof. The neighborhood is usually a prefixed set of nearby, prior pels. For example, in a class of bi-level pictures, the survey may ascertain the relative frequency that a black pel follows a particular neighborhood. Exemplary of a bi-level picture is a typed manuscript. Bi-level facsimile systems allow a descriptive convenience arising in part because bi-level brightness levels may be coded and explained in terms of a single bit for each pel, typically a logic zero or logic one signal for coding a black or white brightness level, respectively. As a consequence of the survey, given a neighborhood of a pel, the most likely brightness level is assigned as a unique prediction of the current pel. In the prior art, the prediction is compared with the current pel signal. If the prediction and pel signal are identical, then a first logic signal, e.g., a logic zero, is transmitted to the receiver; otherwise a second logic signal, e.g., a logic one, is so transmitted. Inasmuch as the prediction is usually correct, a sequence of logic zeros is transmitted. The resultant repetition of like signals, known in the art as a run, leads to inefficient use of the transmission link between transmitter and receiver. To mitigate against the inefficient use, various run-length coding arrangements are known. A typical run-length coder extends two quanta of data to the receiver: one, the brightness level and the other the length of the run, e.g., a count of the number of sequential picture elements having the same brightness level. Of course, a run of length one is possible; however, as the run-length increases, more efficient use of the transmission link is possible.
Accordingly, an object of our invention is to increase the length of a run in a facsimile system.